For non-experienced user

Close down outlook completely

1
Control Panel > (view by icons, not categories) > mail > Show Profiles > Add > (call it whatever you like) > change the dropdown to the new one you just created
This will recreate the profile settings and you will need to re-add your mailboxes

2
C:\users\(you username)\appdata(this is hidden so make sure you can see hidden files - google guides available for this)local\microsoft\outlook
eg
C:\users\BEN\appdata\local\microsoft\outlook
This file, rename it to "Outlookold"
(this backs up the file so we can get it back by changing the name back to "Outlook" if need be - if the fix works, delete this to save yourself some space after you're done)



3
NOW, this is the first time you should be trying to open outlook - it will take a while but should load up after recreating the configuration folder and the configuring your account - then just re-add your mailbox(es).




For experienced users
*insert disclaimer about being careful with RegEdit*

reg delete "\\%asset%\HKEY_USERS\%SID%\Software\Microsoft\Office\16.0\Outlook\Profiles\Outlook"



For super users

*insert disclaimer about being careful with RegEdit*
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Profiles
locate the mail profile that's active
go to 9375CFF0413111d3b88a00104b2a6676
Find which one has the affected mailbox in "Account Name"
Note the "Service UID" - go to the binary value in the "Profile" key that we're in
in here, find the REG_BINARY for "01023d0d" and make a note of the binary - go to the binary value in the "Profile" key that we're in
Right Click > New > Binary Value

Name = 00036619
Value = 01 f0 00 80

M$ is a sack of poo. I can't believe nobody has come up with a professional mail, calendar and contacts package to rival this sack of horse sh*t. The button was already unchecked so this did not help with a recurring problem.